640 EVERYDAY SCIENCE 



of an electric current depends on the size of the current (amperage), 

 and the "pressure," or voltage. This power is measured in watts. 

 The number of watts may be determined accurately for one kind 

 of current and approximately for the other by simply multiplying 

 the number of amperes by the number of volts. For example, an 

 electric iron using a current of 5 amperes under pressure of 110 

 volts requires 550 watts of electrical energy to keep it heated. If 

 this iron is used for an hour, we say that it consumes 550 watt- 

 hours of current. 



But a watt-hour indicates so small an amount of current that 

 the commercial unit of measurement is the kilowatt-hour, 1000 

 watts for an hour's time. Another way of putting it is that 1 kilo- 

 watt-hour = 1000 watt-hours. 



Your electric fixtures are marked with the number of amperes 

 and volts necessary to run them. The iron mentioned above would 



KILOWATT HOURS 



FIGURE 20. DIAL OF A WATT-HOUR METER. 



be marked "5 amperes, 110 volts." In an hour's time this would 

 consume 550 watt-hours of current, as has been shown. This is 

 iVirk, or .55, kilowatt-hour. If your company charges 10 i a kilo- 

 watt-hour, it costs you .55X$.10, or $.055, to operate your electric 

 iron for an hour. 



An electric stove with all the switches open requires an electric 

 stream of about 20 amperes. On a 110-volt current such a stove 

 in full operation would consume in an hour 2200 watt-hours of 

 current. This is MH, or 2.2, kilowatt-hours. At 10 i a kilowatt- 

 hour, such an electric stove, with all the "burners" going, would 

 cost 2.2X$.10, or $.22 an hour. 



